The present invention is concerned generally with a method and apparatus for selectively extracting strontium values from acidic aqueous solutions. More particularly, the invention is concerned with a method and apparatus for selectively extracting strontium and technetium values from biological, commercial waste and environmental samples by use of a chromatographic column.
Growing public health and safety concerns over the use of nuclear technology, both in the production of power and in the manufacture of nuclear weapons, have made increasingly important reliable methods for the determination of various radiostopes in environmental and biological samples. The beta emitting isotope .sup.90 Sr, produced by nuclear fission, is of particular concern. Because of its chemical similarity to calcium, .sup.90 Sr released to the environment can quickly enter the food chain. Ultimately, the .sup.90 Sr can be permanently incorporated into the inorganic matrix of human bones. There is, therefore, a clear need for an analytical procedure and methodology suitable for use in the routine monitoring of persons whose activities expose them to the risk of internal contamination from this element and for the determination of the levels of radiostrontium in various environmental samples (e.g., soils, plants, natural waters, and waste streams).
Numerous prior methods have been described for performing such determinations. An essential feature of all of these methods is the separation and purification of the strontium, both to remove radiosotopes which may interfere with subsequent beta counting and to free it from the large quantities of inactive substances typically present, particularly calcium.
Typical past methods of separation of Ca and .sup.90 Sr have relied upon the greater solubility of calcium nitrate in strong nitric acid solutions. This approach ultimately provides satisfactory results but requires numerous process steps, and repeated precipitations must be performed in order to achieve substantially complete recovery of .sup.90 Sr. Other methods of separation involve precipitation of strontium sulfate or strontium rhodizonate, sorption of strontium on an ion exchange resin from a solution of a chelating agent such as CYDTA or EDTA, and liquid-liquid extraction with organophosphorous acids. These methods, however, suffer from serious shortcomings. In the case of the precipitation and extraction methods, the extracted strontium is contaminated with significant amounts of calcium. Ion exchange processes require precise control of pH due to the narrow range of pH necessary to achieve adequate separation. Moreover, the acceptable pH extraction range is affected by the concentration of calcium present which gives rise to different pH ranges needed for effectively treating different waste samples.
In the recent past, attempts have been made to use macrocyclic polyethers to separate strontium and calcium. Such work has involved a chloroform solution of dicyclohexano-18-Crown-6 to separate strontium traces from macroscopic amounts of calcium ions. This method has also been applied to the determination of the relative amounts of .sup.89 Sr and .sup.90 Sr in milk. In another related process a combined procedure involving ion-exchange/extraction/precipitation has been developed which incorporates the extraction of strontium picrate into a methylene chloride solution of dicyclohexano-18-Crown-6 to isolate traces of strontium from milk and drinking water. In a related process involving chromatographic separation of alkaline earths, solutions are used consisting of various polyethers in chlorinated hydrocarbons dispersed onto siliconized kieselguhr as a stationary phase. Separation of calcium and strontium proceeds by use of either dibenzo-18-Crown-6 or dicyclohexano-18-Crown-6. Unfortunately, each of these Crown ether based strontium extraction procedures is wholely ineffective for relatively acidic samples. This is a particularly troublesome shortcoming since it is often most desirable to digest biological or environmental samples or leach the raw sample using an acid to produce a feed solution for extraction, and such a feed solution is often several molar in acid.
It is therefore an object of the invention to provide a novel method and apparatus for separating strontium or technetium values from biological, commercial waste and environmental samples.
It is another object of the invention to provide an improved method and composition of matter for extracting strontium or technetium from various waste material samples.
It is a further object of the invention to provide a novel method of preparing an extractant media for a liquid chromatographic column based on a solution mixture of a diluent and a Crown ether dispersed on a resin substrate material.
It is still another object of the invention to provide an improved method and apparatus for extracting strontium or technetium dissolved in a strongly acidic solution.
It is an additional object of the invention to provide a novel method of chromatographic extraction of strontium and technetium values from aqueous nitric acid solutions having up to about 6M nitric acid concentrations.
It is yet a further object of the invention to provide an improved method for enhanced extraction of strontium or technetium using a sample loading and washing solution consisting of 2MHNO.sub.3 /0.5M Al(NO.sub.3).sub.3.